The present invention relates to methods and apparatus to effect non-contact measurement of the volume of an object. The invention particularly concerns an apparatus and method for use in an inspection system, for monitoring the content of package and optionally rejecting packages falling outside acceptable volumetric parameters.
In many production processes, inspection of the product is carried out at various stages during the production process, and particularly immediately prior to final packaging and dispatch of the product. In processes for the commercial production of foodstuffs, for example, there is a requirement to ensure that each package of a prepared foodstuff contains the proper amounts of the various components of the foodstuff, prior to its shipping from the factory. As an example, in the production of ready-to-prepare meals comprising a number of pouches or sachets which are packaged in an outer wrapper, it is necessary to inspect the packages to determine that all of the pouches or sachets are present.
In a conventional installation for inspecting products such as foodstuffs, prepared meats, or other materials permable to x-rays, there is provided a conveyor for transporting the packaged products. Above the conveyor a source of x-rays directs a wide beam of x-rays onto the conveyor surface, the x-ray beam being shaped by an aperture plate to form a narrow irradiation zone extending across the width of the conveyor, Beneath the conveyor, and aligned with the irradiation zone, a linear array of photodiodes is arranged to extend across the width of the conveyor. A phosphorescent strip is mounted above the array of photodiodes to extend transversely to the direction of the conveyor, so that x-rays from the source pass between the aperture plates, through the product and the conveyor, and strike the phosphorescent strip. Each point along the length of the phosphorescent strip emits visible light in proportion to the strength of the x-radiation striking the strip at that point, and this visible light is converted by the array of photodiodes into electrical signals. The signal from each photodiode represents the strength of the x-ray beam at the corresponding point across the width of the conveyor.
The thickness and density of the product modulates the intensity of the x-radiation striking the photophorescent strip, and thus modulates the amount of visible light emitted at each point along the length of the phosphorescent strip. The array of photodiodes detects this modulated light emission, and by repeatedly sampling the outputs of the individual photodiodes in the linear array of photodiodes, the product is scanned as it passes through the irradiation zone. The outputs of the photodiodes are conventionally displayed as a video image of the passing product.
In the case of prepared meat products, any bones remaining in the meat will resist penetration of x-rays to a greater extent than will the meat, and thus the photodiode which falls in the xe2x80x9cshadowxe2x80x9d of the bone will be illuminated to a lesser extent than will photodiodes which receive x-rays passing through the meat. The presence of any bone or other body more resistant to x-rays can be detected in the video image as a dark area. The product concerned may then either be re-processed or discarded from the production line.
In an alernative use for the inspection equipment, detection of the absence of a product may be effected. For example, in the final inspection of multiply packaged food items such as cakes or pies, the packages may pass through the irradiation zone and the photdiode outputs are used to form a video image of the packaged items. By monitoring the image, the number of items present within the package can be verified, since a missing item result in a lighter image area than would otherwise be expected. The package may then be rejected.
In the simple detection processes described above, the detected light level is compared by the operative monitoring the video display with a predeterminced xe2x80x9cideal imagexe2x80x9d, and a decision is made on the basis of whether the image is too dark, when foreign bodies are to be detected, or the image is too bright when the absence of an inspected item is to be detected.
Detection in either case is thus dependent on the reliability of the operative monitoring the video display, and difficulties with inspection equipment of this type frequently occur if the operative is distracted or suffers a lapse of concentration.